1. Field of the Invention
This invention relates to an active material for positive electrode in a non-aqueous electrolyte secondary battery, and to the non-aqueous electrolyte secondary battery that uses the active material, and particularly to increasing the capacity, improving the coulomb efficiency, reducing the irreversible capacity, improving the low-temperature output characteristics and improving the cycle characteristics of the non-aqueous electrolyte secondary battery.
2. Description of the Related Art
In recent years, it has become possible to manufacture lithium secondary batteries having high voltage and high energy density. Therefore, due to their compactness and high capacity characteristics, they have been used as the power supply for small portable devices such as mobilephones (cellular phones), notebook computers, camcorders, personal digital assistants (PDA) and the like, and have rapidly penetrated society. Furthermore, recently, research and development has been carried out in order to use them in automobiles, such as in hybrid cars. At present, in society, there is a demand for batteries that have even higher capacity as well as have excellent safety and output characteristics.
Lithium secondary batteries are capable of achieving high voltage and high energy density, and a lithium-cobalt composite oxide is used most often for the positive electrode for the batteries. Much research and development has been carried out for lithium secondary batteries that use this kind of lithium-cobalt composite oxide in order to obtain excellent initial capacity characteristics and cycle characteristics, and various results have been obtained and production is advancing.
However, the lithium-cobalt composite oxides use expensive cobalt compounds as one of the raw materials, which causes an increase in the cost of the positive electrode and thus an increase in the cost of the secondary battery, so there is a large demand for use of a less expensive active material in its place.
Research is being performed for using a lithium-metal composite oxide that uses a metal selected from the group of manganese and nickel as the active material for positive electrode to be used in the place of the lithium-cobalt composite oxide. Particularly, the lithium-nickel composite oxide displays the same high battery voltage as the lithium-cobalt composite oxide, and its theoretical capacity is higher than that of the lithium-cobalt composite oxide, and the raw material nickel is inexpensive and stable in supply, compared to cobalt, so there is much expectation for it as a next-generation active material for positive electrode, and much research and development is being performed.
In the past, when the lithium-nickel composite oxide obtained by a proposed manufacturing method is used for the active material for positive electrode, both the charge capacity and discharge capacity are high compared with the lithium-cobalt composite oxide, and in addition its cycle characteristics are improved. However, there is a problem in that the discharge capacity is less than the charge capacity although in the first charge and discharge only, so that the irreversible capacity, which is defined as the difference between the charge capacity and the discharge capacity is much larger than that of the lithium-cobalt composite oxide.
Moreover, when the lithium-nickel composite oxide is used in a high-temperature environment or low-temperature environment, there is a drawback in that it is comparatively easy to lose battery performance. As a secondary battery, output characteristics at high temperature or low temperature are very important characteristics when the battery is installed in a device that will be used in an environment with large temperature changes, and particularly when used in a cold areas, it is necessary that there be sufficient output characteristics at low temperatures. Therefore, when using a secondary battery that uses the lithium-nickel composite oxide in an automobile, improvement of the low-temperature output characteristics is very important.
In Japanese Patent Publication No. Tokukai Hei 8-7894, a non-aqueous lithium secondary battery that uses LiNiO2 as the active material for positive electrode is proposed, where by using LiNiO2 particles having a secondary particle size in the range of 3 μm to 30 μm, and where 80% or more of the pore volume has a pore radius of 50 nm or less, and where the average pore radius is in the range of 3 nm to 10 nm, so that it is possible to maintain reproducibility of the initial capacity and to secure good cycle characteristics in the active material for positive electrode.
As disclosed in Japanese Patent Publication No. Tokukai Hei 11-185755, in the case of Li Ni1-xCoxO2 (0<x<1), when making an active material for positive electrode from nickel salt, cobalt salt and lithium compound, high initial discharge capacity and good cycle characteristics could be obtained in the active material for positive electrode by strictly controlling the manufacturing conditions. Also, inventions have been proposed that aim at controlling the physical properties of the particles of the active material for positive electrode to improve performance, however it cannot be said that they sufficiently solve the problems mentioned above.
It is proposed that different elements are added to the lithium-nickel composite oxide for replacement with some elements in the composite oxide in order to improve the cycle characteristics (for example, in the disclosure of Japanese Patent Publication No. Tokukai Hei 8-78006, one or more elements selected from the group of B, Al, In and Sn were added to the Li(Ni, Co)O2 composite oxide). The cycle characteristics are improved, however the range for intercalation and deintercalation of the lithium ions of the active material becomes narrow, and there is a tendency for a drop to occur in the discharge capacity. And, this drop in discharge capacity becomes large particularly under heavy load conditions where the discharge current is outstandingly large, or low-temperature efficiency discharge conditions where the lithium ion mobility in the electrolyte becomes low at low temperature.
Also, in the disclosure of Japanese Patent Publication No. Tokukai 2000-30693, in the case of the lithium-metal composite oxide of a hexagonal crystal system having a layer structure and expressed as [Li]3a[Ni1-x-yCoxAly]3b[O2]6c (where the subscripts added to the brackets [ ] indicate the site, and where x and y satisfy the conditions 0<x≦0.20, 0<y≦0.15), the site occupancy rate of metal ions (hereafter referred to as non-lithium ions) other than the lithium at site 3a, obtained from the Rietveld analysis results of X-ray diffraction, is 3% or less, and an active material for positive electrode can be obtained by controlling the crystallite diameter that is calculated from the particle shape and the half width of peak 003 of the X-ray diffraction pattern, so as to make a non-aqueous electrolyte secondary battery having a high initial discharge capacity and small irreversible capacity.
Also, as disclosed in Japanese Patent Publication No. Tokukai Hei 11-224664, a lithium-metal composite oxide having a structure with Co, Mn, Fe, Mg or Al uniformly mixed in solid solution into the crystal structure of the lithium-nickel composite oxide is provided, and by forming a film made of at least one water repellent material selected from the group of a polymer compound containing fluorine and ran organic silicone compound on the surface of particles of lithium-metal composite oxide, and/or the surface of the positive electrode containing such a lithium-metal composite oxide to prevent drops in battery performance due to the effect of moisture on the lithium-metal composite oxide of the positive electrode, and in addition, the process is conducted in work areas such as a dry room in which dehumidification equipment is installed, whereby it is possible to provide a lithium secondary battery that is safe, and has excellent water resistance. However, since the positive electrode or active material for it is coated with a water repellent material, the intercalation and deintercalation of the lithium ions is affected, and it is difficult to achieve highly efficient discharge characteristics.
Also related to this invention, in Japanese Patent Publication No. Tokukai Hei 9-245898, a lithium-metal composite oxide having the composition LixMyO2 (where x is 0.3 to 1.2, y is 0.8 to 1.2, and M is a transition metal) is provided, and by using the lithium-metal composite oxide as an active material for positive electrode for a lithium secondary battery and by controlling the amount of the sulfate radical (SO4) in the composite oxide in the range of is 0.1 weight % to 2.0 weight %, corrosion of the positive electrode assembly of the lithium secondary battery is prevented while at the same time high battery capacity is maintained in the lithium secondary battery obtained. The sulfate radical (SO4) is contained by adding sulfate material before sintering of the composite oxide, or from the sulfate residue which occurs when combining the lithium material with the transition metal material.
Also related to this invention, in Japanese Patent Publication No. Tokukai 2000-21402, a lithium-metal composite oxide that has a composition LixM1-yNyO2-zXa (where M is Co or Ni, N is a transition metal element that is not the same as M or is one or more element selected from the group of the second, thirteenth, or fourteenth group of the periodic table, X is an element of halogen, and 0.2<x≦1.2, 0≦y≦0.5, 0≦z≦1 and 0≦a≦2z) is provided, and a non-aqueous electrolyte or lithium secondary battery having excellent cycle life is obtained by using the lithium-metal composite oxide as the active material for positive electrode with a sulfate radical, based on inorganic and/or organic sulfate, contained in the lithium-metal composite oxide.
Also, in Japanese Patent Publications No. Tokukai 2002-15739 and No. Tokukai 2002-15740, by having at least one element from the group of Na, K, Rb, Cs, Ca, Mg, Sr and Ba coexist with sulfate ions in the lithium-metal composite oxide used for the positive electrode, a lithium secondary battery is obtained that has a high high-temperature capacity maintenance factor, and excellent cycle characteristics even when high-voltage charging is performed. Each of the publications focuses on sulfate ions in order to improve the cycle characteristics. The sulfate ions are added at an appropriate specified location in the manufacturing stage of the battery.